A considerable amount of literature has been published on the demodulation of signals, and numerous developed and commercialized products exist on the market for data rates running from several tens of bit/s (telegraph signals) to several hundreds of Mbit/s (digital connections over cables carrying digitized telephone channel multiplexes at up to 140 Mbit/s or even 560 Mbit/s, and time division multiple access (TDMA) connections at 120 Mbit/s in the Intelsat and Eutelsat satellite systems).
Prior art products all make use of circuits specifically adapted to the functions to be performed (reconstituting the carrier, reconstituting the clock rate, circuits for taking decisions after post-detection filtering, . . . ). These embodiments in analog form or in the form of a transposition of conventional analog concepts all make use of the principle of real time processing which consists of storing only the results of the processing. Original signal storage occurs only in differential demodulation, and even then the storage is applicable for a fixed delay only.
This processing principle has deleterious consequences on performance and on complexity.
Performance is degraded since, when applying this principle, best adaptation to variability in parameters is not achieved and as a result the system is more sensitive to disturbing signals (noise, interfering spectrum line) superposed on the signals to be measured.
Complexity is increased since, when this principle is used, it is very difficult to provide best adaptation to each of the stages which must be implemented in order to achieve the desired object. In particular, the stages of acquisition and of tracking either require different devices, or else they require analog circuit characteristics which are adapted to each of the stages and it is very difficult to exploit and adapt the devices used to each specific stage as well as possible without excessive complication.
As for demodulation, acquisition of the clock and the phase characteristics of the carrier require a great deal more calculation than do tracking and signal demodulation. A conventional solution cannot optimize the processing member to both of these different situations.
Traditional demodulation methods use various phase lock methods for recovering the carrier and the clock (phase locking after squaring, Costas loop, . . . ) and digitizing occurs only when implementing particular technologies, or during linear approximations that require a great deal of calculation.
The method of the invention makes considerable use of addressable digital memories and of software, thereby eliminating all of the above-mentioned difficulties and making it possible to use an overall algorithm and to approach optimum performance (tuned filter) with a minimum of calculation.